1. Field of the Invention
This invention relates to a method of removing ferrous sulfide deposits from ferrous metal surfaces, and more particularly to a method for removing such deposits with an acidic solution containing a selected additive whereby the presence of free hydrogen sulfide is substantially minimized.
2. Description of the Prior Art
In many processes involving sulfur, deposits including ferrous sulfide (FeS) tend to accumulate or build upon ferrous metal surfaces such as reactor walls, piping, and other surfaces. Petroleum refineries, which process crude oil or natural gas, end up with substantial amounts of ferrous sulfide on the metal surfaces of apparatus in contact with the crude oil or gas. The ferrous sulfide which accumulates upon the ferrous metal surfaces commonly is referred to as "scale". The scale must be periodically removed from the metal surfaces in order to restore efficient operation of the scale-coated apparatus.
Numerous techniques previously have been proposed to effect the removal of ferrous sulfide. One method of removing ferrous sulfide comprises contacting the ferrous sulfide with a conventional acid cleaning solution. The acid cleaning solution reacts with the ferrous sulfide and produces gaseous hydrogen sulfide (H.sub.2 S).
Hydrogen sulfide gas produced during the acid cleaning of the surface containing the ferrous sulfide causes environmental and physical problems. First, hydrogen sulfide is an extremely toxic gas and cannot be directly vented to the atmosphere. In addition, hydrogen sulfide and acid cleaning solutions containing hydrogen sulfide can cause severe corrosion on ferrous metals which the solution contacts.
In an effort to avoid the problems associated with the cleaning of ferrous sulfide with an acid, inhibiting compositions of various types have been added to the acid cleaning solutions which react with the hydrogen sulfide and thus prevent the release of the hydrogen sulfide to the atmosphere. One problem associated with this method of control of hydrogen sulfide generation is that many times precipitates form in the cleaning solution and are deposited on the surfaces which are being cleaned.
In another method of cleaning ferrous sulfide scale from metal surfaces, a chelating agent is added to the cleaning solution at a pH such that the hydrogen sulfide is not released to the atmosphere but is retained in the solution as sulfide or bisulfide ions. A major problem associated with this method of cleaning ferrous sulfide scales is that high temperatures are required for the effective operation of the chelating agent and the chelating agents are very expensive.
The present invention provides a method of removing ferrous sulfide deposits from ferrous metal surfaces which overcomes or at least mitigates the above described problems.